For safe control in piloting an aircraft, it is necessary to maintain the control surfaces such as the flap panels and slats of the wings on opposite sides of the aircraft symmetrically. Asymmetric deployment of the control surfaces causes an imbalance in the lift forces applied to opposite sides of the aircraft, thereby tending to throw the aircraft into an uncontrollable state. Prior actuator systems have avoided this problem by sensing the relative positions of the control surfaces on opposite sides of the aircraft and braking against further movement of the control surfaces when the positions of the control surfaces start to become asymmetric as a result of a failure in normal operation of the actuator system.
In one prior system such as is disclosed in U.S. Pat. No. 3,986,689, a flap panel on each wing may be advanced or retracted relative to the wing through the use of one or more ball-screw mechanisms. A power drive unit in the aircraft connecting with each of the ball-screw mechanisms may be actuated under pilot control to either advance or retract the flaps to achieve either greater lift or drag as may be desired by the pilot of the aircraft.
Torque to actuate the ball-screw mechanisms normally is transmitted through a primary load path connecting between the power drive unit and the ball-screw mechanism. In the event that there is a failure in the primary load path, a backup load path connecting between outer ends of the primary path is available, as in Maltby Patent No. 3,986,689, to deliver power to the ball-screw mechanisms so that symmetrical positioning of the control surfaces of the aircraft may still be achieved. Connection to the opposite ends of the primary load path, enables torque to be transmitted through the backup shaft from either end. Accordingly, should there be a failure for example in a middle segment of the primary power path on one side of the aircraft, torque will be transmitted through the shaft of the backup load path from the opposite side of the aircraft to the point of failure thereby providing power to all of the remaining ball-screw mechanisms on the one side from the opposite direction.